High voltage lithium ion positive electrode material

ABSTRACT

A positive electrode material having a nominal stoichiometry Li 1+y/2 Co 1−x−y−z−d Si z Fe x M y M′ d (PO 4 ) 1+y/2  where M is a trivalent cation selected from at least one of Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′ is a divalent cation selected from at least one of Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0&lt;y≤0.10 and x is within a range of 0≤x≤0.2. The use of double compositional modification to LiCoPO 4  increases the discharge capacity from ˜100 mAh/g to about 130 mAh/g while retaining the discharge capacity retention of the singly Fe-substituted LiCoPO 4 . Additional compositional modification to include Si increases the cycle life and greatly improved the coulombic efficiency to between 97-100% at a C/3 cycle rate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 15/099,821 filed Apr. 15, 2016; which is acontinuation application of U.S. patent application Ser. No. 14/281,924filed May 20, 2014, now U.S. Pat. No. 9,356,291 issued May 31, 2016;which claims the benefit of U.S. Provisional Patent Application No.61/911,700 filed on 4 Dec. 2013, the complete disclosures of which, intheir entirety, are herein incorporated by reference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the United States Government.

FIELD OF INTEREST

The present invention relates in general to a positive electrodematerial, and in particular to a high voltage lithium ion positiveelectrode material.

BACKGROUND

LiFePO₄ is a Li-ion positive electrode material that has been favoredfor its strong abuse tolerance which in turn has been attributed to thenature of the oxygen bonding in the phosphate group. Also, it isdesirable to harness the abuse tolerance of a phosphate based cathode ina material that stores more energy. One possibility is to look to highervoltage olivines such as LiMnPO₄ 4.1 V, LiCoPO₄ 4.8 V or LiNiPO₄ 5.1Vsince the stored energy is proportional to the voltage. LiCoPO₄ inparticular has the potential to increase energy 40% compared to LiFePO₄.Furthermore, its electronic structure is more favorable than LiMnPO₄ andLiNiPO₄ in terms of polaronic conductivity and ability to form polarons,respectively.

However, and even though initial research on LiCoPO₄ led to improvementsin the rate of discharge, capacity fade has blocked further progress .As such, an improved Li-ion positive electrode material with a highdischarge capacity and low capacity fade would be desirable.

SUMMARY

A Li-ion positive electrode material is provided. The material has anominal stoichiometry ofLi_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M isa trivalent cation such as Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′is a divalent cation such as Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y iswithin a range of 0<y≤0.10, x is within a range of 0≤x≤0.2, z is withina range of 0≤z≤0.1 and d is within a range of 0≤d≤0.20. In someinstances, d is within the range of 0≤d≤0.10, and preferably within therange of 0≤d≤0.05. The Li-ion positive electrode material has an initialcapacity of at least 120 mAh/g and a discharge capacity of at least 100mAh/g after 500 cycles.

In some instances, the positive electrode material has a compositionwhere y is within the range of 0.02≤y≤0.08, xis within the range of0.05≤x≤0.15 and M=Cr or Ti. In other instances, y is within the range of0.04≤y≤0.06, x is within the range of 0.08≤x≤0.12 amd M=Cr or Ti. Instill other instances, y=0.05, x=0.10 and M=Cr or Ti.

In one embodiment, z and d are equal to zero, and the Li-ion positiveelectrode material has a nominal stoichiometry ofLi_(1.025)Co_(0.85)Fe_(0.1)Cr_(0.05)(PO₄)_(1.025), an initial capacityof at least 125 mAh/g and a discharge capacity of at least 105 mAh/gafter 500 cycles.

In another embodiment, d is equal to zero, z is not equal to zero, thepositive electrode material contains Si and the Li-ion positiveelectrode material has a nominal stoichiometry ofLi_(1+y/2)Co_(1−x−y−z)Si_(z)Fe_(x)M_(y)(PO₄)_(1+y/2) where x and y havethe values given above and z is within a range of 0<z≤0.1, preferablywithin a range of 0<z≤0.05, and more preferably within a range of0<z≤0.02. In some instances z=0.01. Also, the addition of Si improvesthe coulombic efficiency of the material and in some instances thecoulombic efficiency is between 97-100% at a C/3 cycle rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical plot of discharge capacity as a function of cyclenumber for samples of compositionLi_(1+y/2)Co_(1−x−y)Fe_(x)M_(y)(PO₄)_(1+y/2) with M=Cr, Ti, Al, and Ga,and where 10Ti10Fe representsLi_(1.05)Co_(0.80)Fe_(0.10)Ti_(0.10)(PO₄)_(1.05), 5Ti10Fe representsLi_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025), etc.;

FIG. 2 is a graphical plot of discharge capacity as a function of cyclenumber for materials according to an embodiment of the presentinvention;

FIG. 3 is a graphical illustration of long term discharge capacity as afunction of cycle number for materials according to an embodiment of thepresent invention;

FIG. 4 is a graphical plot of X-ray powder diffraction patterns forLi_(1.05)Co_(0.08)Fe_(0.10)Cr_(0.10)(PO₄)_(1.05) (Top) andLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) (Bottom), with thepeaks labeled with the Miller indices of the phospho-olivine structure,Pnma spacegroup.

FIG. 5 is a graphical plot of X-ray powder diffraction patterns ofLi_(1.05)Co_(0.08)Fe_(0.10)Ti_(0.10)(PO₄)_(1.05) (Top),Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(hd 0.05)(PO₄)_(1.025) (Middle) andLi_(1.0125)Co_(0.875)Fe_(0.10)Ti_(0.025)(PO₄)_(1.0125). (Bottom), withthe peaks labeled with the Miller indices of the phospho-olivinestructure, Pnma spacegroup;

FIG. 6 is a graphical plot of voltage versus discharge capacity ofLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) as a function ofrate;

FIG. 7 is a graphical plot of capacity and coulombic efficiency of: (A)Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025); versus (B)Li_(1.025) Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025);

FIG. 8 is a graphical plot of capacity and coulombic efficiency of: (A)Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025); versus (B)Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025);

FIG. 9 is a graphical plot of an X-ray powder diffraction pattern ofLi_(1.025) Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025); and

FIG. 10 is a graphical plot of an X-ray powder diffraction pattern ofLi_(1.025)Co_(0.84)S1 _(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025).

DETAILED DESCRIPTION

An improved Li-ion positive electrode material with an initial capacityof at least 120 mAh/g and a discharge capacity of at least 100 mAh/gafter 500 cycles is provided. In some instances, the improved Li-ionpositive electrode material has initial capacity of at least 125 mAh/gand a discharge capacity of at least 105 mAh/g after 500 cycles. Inaddition, the material can contain Si which provides an coulombicefficiency to between 97-100% at a C/3 cycle rate.

It is appreciated that the improved capacity and dramatically reducedcapacity fade is striking relative to LiCoPO₄. The use of a doublecompositional modification increases the discharge capacity from 100mAh/g to about 130 mAh/g in the most favorable cases for Ti and Fe or Crand Fe modification of LiCoPO₄, while retaining the discharge capacityretention of the singly Fe-substituted LiCoPO₄. Additional compositionalmodification to include Si increases the cycle life and greatly improvesthe coulombic efficiency to between 97-100% at a C/3 cycle rate.

The material has a nominal stoichiometry ofLi_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M isa trivalent cation such as Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′is a divalent cation such as Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y iswithin a range of 0<y<0.10, x is within a range of 0<x<0.2, z is withinthe range of 0≤z≤0.1 and d is within the range of 0≤d≤0.20. In someinstances, y is within the range of 0.02≤y≤0.08, x is within the rangeof 0.05≤x≤0.15 and M=Cr or Ti. In other instances, y is within the rangeof 0.04≤y≤0.06, x is within the range of 0.08≤x≤0.12 and M=Cr or Ti. Instill other instances, y=0.05, x=0.10 and M=Cr or Ti.

The positive electrode material can also contain Si and the Li-ionpositive electrode material can have a nominal stoichiometry ofLi_(i+y/2)Co_(1−x−y−z)Si_(z)Fe_(x)M_(y)(PO₄)_(1+y/2) where x and y havethe values given above and z is within a range of 0<z≤0.1, preferablywithin a range of 0<z≤0.05, and more preferably within a range of0<z≤0.02. In some instances z=0.01.

In order to better teach the invention but not limit its scope in anyway, a solid state synthesis method for making theLi_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) materialand one or more examples of the inventive material are discussed below.

Samples of Li_(1+y/2) Co_(1−x−y)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) withM=Cr, Ti, Al and/or Ga, 0<y≤0.10 and 0≤x≤0.2 were prepared via a solidstate route. Stoichiometric amounts of Co(OH)₂, LiH₂PO₄, Cr₂O₃, TiO₂,Al(OH)₃, Ga₂O₃, FeC₂O_(4.2)H₂O and/or acetylene black (5 wt. % of finalproduct) were ball-milled for 90 minutes. The mixture was then heated ata heating rate of 10° C. min⁻¹ to 700° C. under flowing N₂ and then thereactant mixture was held at this temperature for 12 h. Samples ofLi_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)M_(0.05)(PO₄)_(1.025) with M=Cr orTi were prepared via the same method using Si(OOCCH₃)₄ as the Si source.

The resulting crystalline phase(s) were identified using X-ray powderdiffraction. X-ray data were collected using a Rigaku Ultima IIIdiffractometer. Lattice constants were calculated from peak positionsusing Rietveld refinement of the pattern collected in a parallel beamgeometry. For electrochemical testing, a composite electrode wasfabricated by a slurry coating method. Using N-methylpyrrolidone (NMP)as solvent, a slurry was used to coat an Al foil substrate to produce acomposite electrode of ca. 80 wt. % active material, 10 wt. %polyvinylidene fluoride (PVDF), 8 wt. % super-P carbon and 2 wt. %conductive carbon nanotube composite (CheapTubes.com). The electrodefilm was cut into small discs with an area of 0.97 cm² and dried underan infrared lamp in air before use. In a dry room (Dew point<−50° C.),Li/active coin cells (Hohsen Al-clad CR2032) were assembled using 3layers of Celgard® 2400 as the separator and a 1.0 molal LiPF₆ solutionin a 3:7 (wt. %) mixture of ethylene carbonate (EC) and ethyl methylcarbonate (EMC) electrolyte with 1 wt. % HFiP. Also, 100-150 μL ofelectrolyte was used per coin cell and electrochemical testing wasperformed using a Maccor Series 4000 tester. For calculation of C-rate,a capacity of ˜150 mA h g⁻¹ was assumed.

Substitution of elements in addition to Fe for Co, including Cr, Ti, Aland Ga, increased the discharge capacity of Fe-substituted LiCoPO₄ whilemaintaining long cycle life. Not being bound by theory, a nominalstoichiometry, Li_(1+y/2)Co_(1−x−y)Fe_(x)M_(y)(PO₄)_(1+y/2), (M=Cr, Ti,Al and/or Ga) that favors substitution on the Co site and cationvacancies exhibited favorable performance since substitution on the Lisite blocks Li-ion conduction. In addition, the above stoichiometryfavors the presence of cation vacancies that might enhance Li-ionconductivity.

Turning now to FIG. 1, capacity as a function of cycle for a number ofdifferent compositions are shown. The Ti,Fe and Cr,Fe doublemodifications exhibited the most favorable discharge capacity and cyclelife. Furthermore, a composition level was discovered to be 0.10 for Feand 0.05 for Ti or Cr. All samples exhibited superior cycle lifecompared to LiCoPO₄ and increased discharge capacity relative to singlyFe-substituted LiCoPO₄—labeled 20Fe(ARL) in the figure and correspondsto nominal stoichiometry of LiCo_(0.8)Fe_(0.2)PO₄).

FIG. 2 demonstrates the effect of Ti,Fe and Cr,Fe double compositionalmodifications on the capacity fade at the optimized dopant level of 0.10Fe and 0.05 Ti or Cr. All samples including the un-substituted LiCoPO4control sample were cycled between 3.5 and 5 V using 1 M LiPF₆ in 3:7 byweight ethylene carbonate: ethyl methyl carbonate electrolyte containing1 wt. % HFiP electrolyte additive. Cells were charged using a C/3constant current to 5 V followed by a constant voltage of 5 V until thecurrent was less than C/15. It is appreciated that the terms “C”, “C/3”,“C/15”, etc., refer to the C-rate used by the battery industry to scalethe charge and discharge current of a battery. For example, a 1,000 mAhbattery discharged at a 1C rate ideally provides a current of 1,000 mAfor one hour, whereas a C/2 discharge rate for a 1,000 mAh batteryideally provides a current of 500 mA for two hours.

As shown in FIG. 2, the un-substituted LiCoPO₄ control sample exhibitedsevere capacity fade. However, the Fe-substitution into LiCoPO₄ (nominalLiCo_(0.08)Fe_(0.2)PO₄) had a significant reduction in capacity faderelative to the un-substituted LiCoPO₄. Also, increasing the dischargecapacity by further composition modification with another element inaddition to Fe is shown. For example, theLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) sample showed bettercapacity retention with cycling despite having a slightly smallerinitial discharge capacity thanLi_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025).

FIG. 3 compares the long term cycling of compositionLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) relative toLiCo_(0.8)Fe_(0.2)PO₄. The Fe-substituted LiCoPO₄ retained about 80% ofits capacity at the 500th cycle (89 of 108 mAh g⁻¹ initial capacity).The Cr and Fe-doubly substituted LiCoPO₄ had an initial capacity higherthan Fe substituted LiCoPO₄, but also retained about 80% of capacity atthe 500^(th) cycle (107 of 131 mAh g⁻¹ initial capacity).

FIG. 4 is a comparison the XRD patterns ofLi_(1+y/2)Co_(0.90−y)Fe_(0.10)Cr_(y)(PO₄)_(1+y/2) where y=0.05 and 0.10.As observed in the lower XRD pattern,Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) forms aphospho-olivine structure without other XRD detectable phases. Thepattern can be indexed to the phospho-olivine structure confirming thata single phase was present. From Rietveld analysis of the X-raydiffraction data, the unit cell volume ofLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) was determined to be282.9 Å³. Also, the lattice parameters are listed in Table 1 below withcomparison to LiCoPO₄ [11] and LiCo_(0.9)Fe_(0.1)PO₄ [12]. The unit cellvolumes of LiCoPO₄ and LiCo_(0.9)Fe_(0.1)PO₄ are 284.3 Å³ and 285.1 Å³,respectively. This decrease in unit cell volume forLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) is consistent withthe substitution of the considerably smaller 6-coordinate Cr³⁺(effective ionic radius=0.615 Å [13]) for Co²⁺ (effective ionicradius=0.745 Å [13]) and Fe²⁺ (effective ionic radius=0.780 Å [13]), andthe presence of cation vacancies which are required to maintain chargeneutrality. The decrease in unit cell volume for the Cr,Fe substitutedLiCoPO₄ results from a decrease in the a and b lattice parameters withlittle change in the c parameter.

Not being bound by theory, improved discharge electrochemicalperformance may likely result from increased electronic and/or ionicconductivity. The Li-ion conductivity is, of course, a function of theLi-ion concentration and the Li-ion mobility. Since there is littledifference in the Li-ion concentration between the modified LiCoPO₄ andthe unmodified LiCoPO₄, the increased Li-ion mobility is a likelyhypothesis for the improvement in discharge capacity and rate capability(rate shown in FIG. 6) of Cr,Fe-substituted LiCoPO₄ relative to LiCoPO₄.

TABLE 1 Nominal composition a (Å) b (Å) c (Å) V (Å³) LiCoPO₄ 10.20485.9245 4.7017 284.3 LiCo_(0.9)Fe_(0.1)PO₄ 10.2175 5.9335 4.7025 285.1Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) 10.1703 5.9204 4.6991282.9 Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) 10.2019 5.92994.6976 284.2 Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025)10.2009 5.9314 4.6999 284.4Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) 10.20605.9308 4.6986 284.4

When a higher Cr content was prepared,Li_(1.05)Co_(0.8)Fe_(0.10)Cr_(0.10)(PO₄)_(1.05), the XRD pattern showedan extra peak around 25 degrees two-theta which matches Li₉Cr₃P₈O₂₉.This is in agreement with the electrochemical results which showed anoptimal level of 0.05 Cr, since at a higher Cr content anon-electrochemically active second phase (Li₉Cr₃P₈O₂₉) appears.

FIG. 5 compares the XRD patterns ofLi_(1+y/2)Co_(0.90−y)Fe_(0.10)Ti_(y)(PO₄)_(1+y/2) where y=0.025, 0.05and 0.10. The pattern matches LiCoPO₄ except for a broad peak atapproximately 24.5 degrees two-theta which can be assigned to aLiTi₂(PO₄)₃-like phase that is present in all the Ti-containing samples.This secondary phase grows as the Ti level increases. A unit cell volumeof Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) was determined tobe 284.4 Å³. The lattice parameters also are listed in Table 1 withcomparison to LiCoPO₄ [11] and LiCo_(0.9)Fe_(0.1)PO₄ [12]. The unit cellvolumes of LiCoPO₄ and LiCo_(0.9)Fe_(0.1)PO₄ are 284.3 Å³ and 285.1 Å³,respectively. The slightly smaller unit cell is consistent with thesubstitution of either Ti⁴⁺ (0.65 Å) or Ti³⁺ (0.67 Å) for Co²⁺ (0.745)[13].

The presence of the secondary LiTi₂(PO₄)₃-like phase suggests that themechanism to increase the electrochemical performance ofTi,Fe-substituted LiCoPO₄ may differ from that of the Cr,Fe substitutedLiCoPO₄. As previously discussed, at a 0.05 Cr substitution level, abulk substitution for Co is obtained. In contrast, the Ti, Fe modifiedLiCoPO₄ samples all contain a small fraction of a LiTi₂(PO₄)₃-like phaseand we therefore suggest that the improvement in electrochemicalperformance for Ti,Fe modified LiCoPO₄ may result from the beneficialeffect this phase has on the Li-ion conductivity of the substitutedLiCoPO₄.

The LiTi₂(PO₄)₃-like phase has the NASICON structure and it is known tobe an excellent Li-ion conductor as a result of the structuralcharacteristics of the NASICON structure which favor high Li-ionicconductivity. Furthermore, the interface of two Li-ion conductingmaterials can lead to orders of magnitude increased Li-ion conductivityon both sides of the interface through a synergistic effect.

Having shown the improvements in capacity fade because of doublecompositional modifications of LiCoPO₄, the favorable effects ofcomposition modification on the capacity ofLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) as a function of rateare shown in FIG. 6. Typical discharge curves at three rates, C/10, C/2and C are plotted. The capacity at C/10 is about 132 mAh/g. At a 1C ratethe capacity is about 126 mAh/g, thus the material shows an excellentrate capability. The improvements are suggested to result from enhancedLi-ion conduction.

In addition to the above, the coulombic efficiency of the nominalcomposition ofLi_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)M_(0.05)(PO₄)_(1.025) with M=Cr orTi was improved by adding Si. The discharge capacity as a function ofcycle and cycle life is shown in FIG. 7 forLi_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) relative toits Si free analog. As shown in the figure, a significant improvement inthe coulombic efficiency and cycle life was observed.Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) showed asimilar improvement in coulombic efficiency and an even greaterimprovement in cycle life as shown in FIG. 8.

The x-ray diffraction pattern of the nominal compositionLi_(1.025)Co_(0.84) Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) is shown inFIG. 9. There is a peak around 25 degrees which does not correspond tothe phospho-olivine structure. This might either be Si related orLi₉Cr₃P₈O₂₉ as previously discussed. The lattice constants (see Table 1)show a decrease in the unit cell volume relative toLiFe_(0.10)Co_(0.90)PO₄, but the unit cell volume is larger thanLi_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025,) and suggests thatinclusion of Si in the starting composition reduces the amount of Crthat can be substituted into LiCoPO₄. However the reduced unit cellvolume relative to LiFe_(0.10)03Co_(0.90)PO₄ indicates that somesubstitution on the Co lattice site is still occurring. From a crystalchemistry perspective, Cr (effective radius=0.615 Å [13]) is more likelyto substitute for Co (effective ionic radius of 0.745 Å [13]) than Si(effective ionic radius of 0.40 Å [13]). The x-ray diffraction patternof nominal compositionLi_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) is shown inFIG. 10. There is a peak around 24.6 degrees two-theta which does notcorrespond to the phospho-olivine structure. This peak might beattributed to a Li₂Si₂O₅, but a definitive assignment was not made sinceonly 1 peak is evident and it falls within the same region as theLiTi₂(PO₄)₃-like phase discussed earlier for Ti containing samples. Theunit cell volume (Table 1) for this sample was found to be 284.41 Å³,indicating no change from the Si free analog within the uncertainty ofthe measurement.

Changes and modifications to the teachings disclosed herein will beobvious to those skilled in the art and yet fall within the scope of thepresent invention. As such, the scope of the invention is defined by theclaims and all equivalents thereof.

1-20. (canceled)
 21. A lithium-ion battery, the battery comprising: apositive electrode, comprising a compound of formulaLi_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄) wherein, M is atrivalent cation selected from at least one of Cr, Ti, Al, Mn, Ni, V,Sc, La, and/or Ga; M′ is a divalent cation selected from at least one ofMn, Ni, Zn, Sr, Cu, Ca, and/or Mg; y is within a range of 0<y≤0.10; x iswithin a range of 0<x≤0.20; z is within a range of 0≤z≤0.10; and d iswithin a range of 0≤d≤0.20; the positive electrode material having aninitial capacity of at least 120 mAhg⁻¹; and a negative electrode; andan electrolyte.
 22. The battery of claim 21, wherein z=0, y is withinthe range of 0.02≤y<0.08 and xis within the range of 0.05<x≤0.15 andM=Cr.
 23. The battery of claim 21, wherein y is within the range of0.04≤y<0.06 and x is within the range of 0.08≤x≤0.12.
 24. The battery ofclaim 21, wherein z=0.
 25. The battery of claim 21, wherein d=0.
 26. Thebattery of claim 21, wherein y=0.05 and x=0.10.
 27. The battery of claim21, wherein 0<z≤0.10.
 28. The battery of claim 21, wherein 0<d≤0.20. 29.The battery of claim 21, wherein x is within the range of 0.08≤x≤0.12.30. The battery of claim 21, wherein y is 0.02≤y<0.08.
 29. A lithium-ionbattery, the battery comprising: a positive electrode, comprising acompound of formula Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)wherein, M is a trivalent cation selected from at least one of Cr, Ti,Al, Mn, Ni, V, Sc, La, and/or Ga; M′ is a covalent cation selected fromat least one of Mn, Ni, Zn, Sr, Cu, Ca, and/or Mg; y is within a rangeof 0<y≤0.10; x is within a range of 0<x≤0.20; z is within a range of0≤z≤0.10; and d is within a range of 0≤d≤0.20; the positive electrodematerial having a capacity of at least 100 mAhg⁻¹ after 500 cycles; anda negative electrode; and an electrolyte.
 30. The battery of claim 21,wherein z=0, y is within the range of 0.02≤y<0.08 and xis within therange of 0.05≤x≤0.15 and M=Cr.
 31. The battery of claim 29, wherein y iswithin the range of 0.04≤y<0.06 and x is within the range of0.08≤x≤0.12.
 32. The battery of claim 29, wherein z=0.
 33. The batteryof claim 29, wherein d=0.
 34. The battery of claim 29, wherein y=0.05and x=0.10.
 35. The battery of claim 29, wherein 0<z≤0.10.
 36. Thebattery of claim 29, wherein 0<d≤0.20.
 37. The battery of claim 29,wherein x is within the range of 0.08≤x≤0.12.
 38. The battery of claim29, wherein y is 0.02≤y<0.08.
 39. The battery of claim 29, wherein thepositive electrode has a coulombic efficiency between 97-100% at a C/3cycle rate.
 40. The battery of claim 29, wherein the positive electrodematerial has an initial capacity of at least 125 mAhg⁻¹.